When charging and discharging are repeated many times, the multiple secondary batteries connected to one another in series cause unbalance in charging voltages of the secondary batteries. Namely, a phenomenon occurs where charging voltages of the respective secondary batteries are not equal to each other. When the charging voltages of the respective secondary batteries become unequal extremely, there exist secondary batteries that are sufficiently charged with high voltage and secondary batteries that are short of charging because of low charging voltages. When the whole multiple series-connected secondary batteries are charged again in order to charge the insufficiently charged secondary batteries, the sufficiently charged secondary batteries are overcharged. When the secondary batteries are overcharged, the life of the barriers becomes short. Moreover, the insufficiently charged secondary batteries are discharged, the insufficiently charged secondary batteries are overdischarged. In the case of overdischarging, discharging cannot be performed any more. Accordingly, when voltage unbalance occurs, not only the entire capacity reduces but also an influence is exerted upon the life of the batteries, so that the ability cannot be satisfactorily exerted as a whole.
In order to prevent such voltage unbalance, a voltage detection circuit that monitors voltage of the secondary battery and a discharge circuit that discharges the secondary battery based on the monitoring result were provided for each secondary battery and there was need to provide a voltage balance circuit shown in FIG. 9.
However, the conventional voltage balance circuit has the following problems.
FIG. 9 is a circuit diagram of a voltage balance circuit of the conventional secondary battery.
A voltage balance circuit 10 is one that balances charging voltages of three secondary batteries B1, B2, and B3 connected to one another in series. The voltage balance circuit 10 includes three Zener diodes 11, 12, and 13. A cathode of the Zener diode 11 is connected to a positive polarity of the secondary battery B1. An anode of the Zener diode 11 is connected to a connecting node N1 between the negative polarity of the secondary battery B1 and the positive polarity of the secondary battery B2. A cathode of the Zener diode 12 is connected to the connecting node N1. An anode of the Zener diode 12 is connected to a connecting node N2 between the negative polarity of the secondary battery B2 and the positive polarity of the secondary battery B3. A cathode of the Zener diode 13 is connected to the connecting node N2. An anode of the Zener diode 13 is connected to the negative polarity of the secondary battery B3.
When the voltage of the corresponding secondary battery B1 exceeds a yield point of the Zener diode 11, current flows into the Zener diode 11 and the secondary battery B1 is discharged. When the voltage of the corresponding secondary battery B1 does not exceed the yield point of the Zener diode 11, no current flows and the secondary battery B1 is charged. The same can be applied to the respective Zener diodes 12 and 13. Namely, when the voltages of the corresponding secondary batteries B2 and B3 are higher than the yield point of the Zener diode 11, current flows into the respective Zener diodes 12 and 13 and the secondary batteries B2 and B3 discharge. When they do not exceed the yield point, no current flows into the respective Zener diodes 12 and 13 and the secondary batteries B2 and B3 are charged. Accordingly, the charging voltages of the secondary batteries B1 to B3 are balanced.
While, in the case of balancing the voltages of the respective capacitors connected to one another in series, a voltage detection circuit that monitors voltage of the capacitor and a discharge circuit that discharges based on the monitoring result are provided for each capacitor. Moreover, there was need to provide a voltage balance circuit as illustrated in next FIG. 10.
A voltage balance circuit 20 is one that balances charging voltages of three capacitors C1, C2, and C3 connected to one another in series. The voltage balance circuit 20 includes three resistors 21, 22, and 23 connected in parallel to the capacitors C1 to C3 respectively. The resistance values of the resistors 21 to 23 are equal to each other. Voltages divided by the resistors 21 to 23 are applied to connecting nodes of the capacitors C1 to C3. Accordingly, charging voltages of the capacitors C1 to C3 are balanced.
In the conventional voltage balance circuit 10 of FIG. 9, current flows into the respective Zener diodes 11 to 13 to prevent the respective secondary batteries B1 to B3 from being overcharged. However, when current flows into tire Zener diodes 11 to 13 simultaneously, loss is generated by the current to reduce efficiency. Moreover, since the charging voltages of the secondary batteries B1 to B3 are decided by breakdown voltage of the respective Zener diodes 11 to 13, there was a case in which the charging voltage varied depending on the accuracy of the Zener diodes 11 to 13.
While, in the voltage balance circuit 20 of FIG. 10, since constant current flows into the resistors 21 to 23, loss is generated.
Moreover, when the voltage detection circuit, which detects the charging voltage, is provided for each of the secondary batteries B1 to B3 or each of the capacitors C1 to C3, the circuit scale is increased.